An antiferromagnetically coupled hexanuclear copper(II) Schiff base complex containing phenoxo and dicyanamido bridges: Structural aspects and magnetic properties

Article abstract of DOI:10.1016/j.poly.2011.03.049

A novel hexanuclear complex [{(CuL)₂Cu}₂(μ-dca) ₂](ClO₄)₂·2L′(1) (where H ₂L = (OH)C₆H₄C(CH₃)N(CH ₂)₃NC(CH₃)C₆H<

Full text of DOI:10.1016/j.poly.2011.03.049

Polyhedron 30 (2011) 1767–1773
Contents lists available at ScienceDirect
Polyhedron
journal homepage: www.elsevier.com/locate/poly
An antiferromagnetically coupled hexanuclear copper(II) Schiff base complex
containing phenoxo and dicyanamido bridges: Structural aspects
and magnetic properties
Pritha Talukder a,1, Shyamapada Shit b, Ashok Sasmal a, Stuart R. Batten c, Boujemaa Moubaraki c,
Keith S. Murray c, Samiran Mitra a,
⇑
a Department of Chemistry, Jadavpur University, Kolkata 700 032, West Bengal, India
b Department of Chemistry, Krishnagar Government College, Krishnagar, Nadia 741 101,West Bengal, India
c School of Chemistry, Monash University, 3800 Victoria, Australia
a r t i c l e i n f o
a b s t r a c t
Article history:
A novel hexanuclear complex [{(CuL)2Cu}2(
l
-dca)2](ClO4)2ꢀ2L0(1) (where H2L = (OH)C6H4C(CH3)@N(CH2)3-
ꢁ
N@C(CH3)C6H4(OH) or N,N0-bis(2-hydroxyacetophenone) propylenediimine and dca = N(CN)2 and L0 = 2-
hydroxyacetophenone) has been synthesized and characterized by elemental analysis, UV–Vis, FT-IR spec-
troscopic and electrochemical methods. Single crystal X-ray structural characterization reveals a centro-
symmetric nature of the complex unit, where the metal centers adopt distorted square-planar and
Received 19 February 2011
Accepted 26 March 2011
Available online 23 April 2011
Keywords:
Hexanuclear complex
Schiff base
Distorted square-planar copper(II)
Distorted octahedral copper(II)
distorted octahedral geometries. Structural analysis also reveals
and the central copper(II) centers of the asymmetric [(CuL)2Cu(dca)] unit, the latter being interconnected
to its symmetry related counterpart by double 1,5-dca bridges, leading to the hexanuclear complex ion
l2-phenoxo bridges between terminal
l
[{(CuL)2Cu}2(l
-dca)2]2+. Low temperature susceptibility measurements of 1 revealed the presence of strong
antiferromagnetic coupling (2J = ꢁ407 cmꢁ1) between the copper(II) centers.
l2-Phenoxo and
l1,5-dca bridges
Ó 2011 Elsevier Ltd. All rights reserved.
Antiferromagnetic
1. Introduction
to study spin frustration and antisymmetric exchange phenomena
[12], as well as for the rational synthesis of ferromagnetic entities
[13,14]. Infinite lattices constitute a further well-studied class of
copper(II) complexes leading to the formation of 1D, 2D and 3D
architectures [15–21], while copper(II) polynuclear complexes
with aesthetically pleasing structures and interesting magnetic
properties are a hot topic in current research [22–27]. Plenty of
polynuclear metal complexes up to four copper centers, with di-
verse structures and detailed magnetic behavior, are known in
the literature, however complexes with more copper centers, in
particular six, are still scarce.
Designing, synthesizing and establishing magneto-structural
correlations of polynuclear copper(II) complexes have been a long
standing interest of our group. With this purpose, our strategies
were to employ different Schiff base coligands for their synthetic
ease, structural flexibility and topological richness in conjunction
with the counter anions of the copper(II) salts and various pseud-
ohalides, e.g. azide, thiocyanate, cyanate and dicyanamide etc., for
their versatile coordination modes, altogether resulting in a wide
variety of topologies and functional materials. Tetradentate ligands
of salen-type Schiff bases with N2O2 donor sets are a long standing
choice of our group as they provided suitable coordination envi-
ronments to form trinuclear complexes with fascinating structures,
depending upon the counter ions of the copper(II) salts used
Polynuclear complexes are of considerable interest for the
development of new magnetic molecular materials [1–3]. Consid-
erable effort has been devoted to the design, synthesis and modifi-
cation of such complexes, obtained by the appropriate choice of
metal ions, ligands and auxiliary ligands, and a detailed examina-
tion of their structure–function correlation helps to develop new
synthetic routes to novel magnetic materials. Studies of such com-
plexes also find wide applications as model compounds for the
investigations of the role of polymetallic active sites in biological
systems [4].
On the other hand, the coordination chemistry of copper(II) has
developed very rapidly over the last four decades due to a number
of reasons. From the standpoint of bioinorganic chemistry, cop-
per(II) coordination complexes have been used for the modelling
of active sites in copper(II) biomolecules [5], while from the per-
spective of magneto-chemistry dinuclear complexes of this ion
have been synthesized to establish magneto-structural correla-
tions [6–11], and trinuclear complexes have got attention in order
⇑
Corresponding author. Tel.: +91 33 2414 6666x2505; fax: +91 33 2414 6414.
E-mail address: smitra_2002@yahoo.com (S. Mitra).
Present address: Department of Chemistry, M.S. Ramaiah Institute of Technology,
1
Bangalore 560 054, India.
0277-5387/$ - see front matter Ó 2011 Elsevier Ltd. All rights reserved.
doi:10.1016/j.poly.2011.03.049

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P. Talukder et al. / Polyhedron 30 (2011) 1767–1773
[28,29]. In our recent paper, we have reported two new tetranu-
clear copper(II) complexes with one of the N2O2 donor Schiff base
ligands used earlier, and we have explored the role of the azide ion
to increase the nuclearity of the complexes [30].
Considering the versatile coordination modes of dicyanamide
(Scheme 1), a larger pseudohalide molecular ion-rod which results
in diverse magnetic materials such as ferro-, antiferro- and canted-
spin antiferromagnetic materials [31–34], our aim was to explore
the possibility of dicyanamide forming a polynuclear complex with
more than four copper(II) centers using same Schiff base ligand
[30], and to investigate its role in the overall magnetic behavior.
In this paper we report a novel hexanuclear copper(II) complex
elemental analyser. The copper content in the complex has been
estimated quantitatively by a standard iodometric procedure. The
FT-IR spectrum (4000–200 cmꢁ1) was recorded on a Perkin–Elmer
Spectrometer RX I FT-IR system with solid KBr pellets. The elec-
tronic spectrum (800–200 nm) was recorded on a Perkin–Elmer
Lambda 40 UV–Vis spectrometer using a 1 ꢂ 10ꢁ5 M solution of
the complex in HPLC grade acetonitrile. Electrochemical studies
were performed on a VersaStat II cyclic voltammeter instrument
using HPLC grade acetonitrile (CH3CN) as the solvent and tetrabu-
tylammonium perchlorate as the supporting electrolyte at a scan
rate of 50 mV sꢁ1, with a platinum wire as the working electrode
and a standard calomel electrode as the reference. Variable-tem-
perature magnetic susceptibility measurements were made at
Monash University using a Quantum Design MPMS 5 SQUID mag-
netometer operating in an applied field of 1 T. The powdered sam-
ples were contained in calibrated gelatine capsules, which were
held at the centre of a drinking straw, the latter being attached
to the end of the sample rod. The temperature and field were
checked against a standard palladium sample (Quantum Design)
of accurately known magnetization and by use of magnetochemi-
cal calibrants such as CuSO4ꢀ5H2O and [Ni(en)3]S2O3. Pascal’s con-
stants were used for diamagnetic corrections.
[{(CuL)2Cu}2(
l
-dca)2](ClO4)2ꢀ2L0 (1) obtained by the reactions of
copper(II) perchlorate hexahydrate, the Schiff base ligand (H2L)
(Scheme 2) and sodium dicyanamide, and characterized by differ-
ent physicochemical methods. X-ray crystallographic characteriza-
tion of
asymmetric [(CuL)2Cu(dca)] units are interconnected by doubly
1,5-dca bridges. Within the [(CuL)2Cu(dca)] unit, terminal cop-
1 reveals its centrosymmetric nature where two
l
per(II) centres are in slightly distorted square-planar geometries,
leaving the central copper(II) to adopt a distorted octahedral geom-
etry. Low temperature magnetic susceptibility studies reveal an
overall strong antiferromagnetic behavior of the complex with
2J = ꢁ407 cmꢁ1, well fitted with the ‘‘pair of independent trian-
gles’’ model [35].
Safety note. Perchlorate salts of transition metal ions in the pres-
ence of organic ligands are potentially explosive, and although we
have not encountered any problems during the synthesis of the
complex, it should be prepared in small quantities and must be
handled with caution.
2. Experimental
2.1. General and physical measurements
2.2. Preparation of the ligand and complex
All manipulations were performed under aerobic conditions
using the materials as received (Aldrich Chemical Co.). The solvents
used were of analytical grade. Elemental analyses (carbon, hydro-
gen and nitrogen) were carried out with a Perkin–Elmer 2400 II
2.2.1. Preparation of the Schiff base ligand (H2L)
2.2.1.1. propylenediimine (H2L). The Schiff base ligand (H2L) was pre-
pared according to the published procedure [29] by refluxing 2-
hydroxyacetophenone (1.361 g, 10 mmol) and 1,3-diaminopropane
N
N
N
N
N
N
N
N
N
N
µ1
µ3
N
N
µ1.5
µ1.3
N
N
N
N
N
N
µ1.1.5
µ1.3.5
N
N
N
N
N
N
µ1.1.3.5
µ1.1.3.5.5
Scheme 1. Different bridging modes of the dca ligand.

P. Talukder et al. / Polyhedron 30 (2011) 1767–1773
1769
CH3
H3C
O
O
+
H2N
NH2
+
HO
OH
CH3
H3C
N
N
OH
HO
H2L
-2H+
CH3
H3C
N
N
O-
-O
L2-
Scheme 2. Synthetic scheme of the Schiff base ligand (H2L) and its coordination modes.
(0.370 g, 5 mmol) in 25 mL methanol solution for an hour. The
resulting solution yielded shiny yellow crystals of the ligand upon
slow evaporation. They were dried and stored in vacuo over CaCl2
for subsequent use. Yield: 1.319 g (85%): Anal. Calc. for C19H22N2O2
(310.39): C, 73.52; H, 7.14; N, 9.03. Found: C, 73.41; H, 7.11; N,
9.01%. IR (KBr pellet, cmꢁ1): 1623 s, 3449 mb, 1204 s. UV–Vis (k,
nm): 241, 371.
and the x scan technique were used at 123(2) K to collect the
intensity data. Data collection and unit cell refinement were car-
ried out using the Bruker XSCANS [36] program. No significant loss
of intensity was observed. A multi-scan absorption correction
was applied to the intensity values (Tmin = 0.5192, Tmax = 0.9284)
empirically using SADABS [37]. Data reductions were performed
using the Bruker SHELXTL [38] program. The crystal structure was
solved by direct methods using the program SHELXS-97 [39],
combined with Fourier difference synthesis, and refined with the
full matrix least square technique based on F2 using the program
SHELXL-97 [39]. All non-hydrogen atoms were refined anisotropi-
cally. All hydrogen atoms were located from the difference Fourier
map and treated with suitable riding models with isotropic dis-
placement parameters derived from their carrier atoms, except
the hydrogen atoms of the methyl and hydroxyl groups which
were refined with isotropic thermal parameters. Molecular graph-
ics and crystallographic illustrations were prepared using ORTEP [40]
and Bruker SHELXTL [38] software. Relevant crystallographic and
refinement data are listed in Table 1.
2.2.2. Preparation of the complex
2.2.2.1. [{(CuL)2Cu}2(
l
-dca)2](ClO4)2ꢀ2L0 (1). To a methanolic solu-
tion (20 mL) of copper(II) perchlorate hexahydrate (1.11 g,
3 mmol), a 10 mL methanolic solution (1.438 g, 2 mmol) of the
Schiff base ligand (H2L) was added, followed by the addition of a
10 mL (0.178 g, 1 mmol) aqueous solution of sodium dicyanamide
with slow stirring. The resulting deep reddish brown solution was
filtered and kept undisturbed at room temperature. After several
days, good quality red brown plate shaped single crystals of 1 sep-
arated out from the mother liquor. These were filtered, washed
with methanol–water mixture (1:1) and air dried. Yield: 68%. Anal.
Calc for C96H96Cl2Cu6N14O20 (2218.07): C, 51.98; H, 4.36; N, 8.84;
Cu, 17.19. Found: C, 51.95; H, 4.32; N, 8.82; Cu, 17.16%. IR (KBr pel-
let, cmꢁ1): 2260 s, 2218 s, 2150 s, 1592 s, 1164 s, 420 w, 365 w.
3. Results and discussion
2.3. Single-crystal X-ray crystallography
3.1. FT-IR spectrum
A red brown plate shaped single crystal of 1 with dimensions
0.075 ꢂ 0.15 ꢂ 0.30 mm3 was mounted on a Bruker P4 diffractom-
The infrared spectrum of 1 was recorded within the 4000–
400 cmꢁ1 region. The spectral data indicate ligand coordination
eter. Graphite-monochromatized Mo K
a
radiation (k = 0.71073 Å)
to the metal. A
strong sharp band observed at 1592 cmꢁ1

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P. Talukder et al. / Polyhedron 30 (2011) 1767–1773
an O(phenolate) ? metal(II) LMCT or a metal(II) ? p⁄ (phenolate)
Table 1
Crystallographic data collection and structure refinement for 1.
MLCT transition], it is accepted that a higher energy component
should exist in the vicinity of 330 nm [47]. Besides these bands
associated to ligand/ligand–metal, the electronic spectrum of 1
also shows two weak and less well defined broad bands at 615
and 675 nm, correlated to the d–d transitions in the copper(II) cen-
ters with square-planar (in agreement with the coordination envi-
ronment of Cu1 and Cu2 in 1) and octahedral geometries (in
agreement with the coordination of Cu3 in 1), respectively [48].
Parameters for
1
Empirical formula
Formula weight
Crystal system
Space group
a (Å)
C
96H96Cl2Cu6N14O20
2218.07
triclinic
P1 (No. 2)
ꢀ
11.2318(1)
14.8201(2)
15.1195(2)
86.858(1)
68.242(1)
78.298(1)
2288.28(5)
1
123(2)
1.610
1.509
1138
b (Å)
c (Å)
a
(°)
b (°)
3.3. Description of the structure
c
(°)
V (Å3)
Z
The molecular structure of 1 consists of the centrosymmetric
hexanuclear complex ion [{(CuL)2Cu}2(l , perchlorate
-dca)2]2+
T (K)
Dcalc (g cmꢁ3
)
counter ions and two 2-hydroxyacetophenone molecules. An ORTEP
view of the asymmetric unit for the complex ion is shown in Fig. 1.
The formation of the complex ion (Fig. 2) may be viewed as a dimer
of [(CuL)2Cu(dca)] unit, each being interconnected with its symme-
l
(mmꢁ1
)
F (0 0 0)
h Range for data collection (°)
Total data
Rint
3.00–27.86
36830
0.0663
try related counterpart through double l1,5-dca bridges. The asym-
Unique data
10794
metric unit consists of three metal ions; two terminal copper(II)
centers [Cu1 and Cu2] with similar coordination environments
and geometries, which differ for the central copper(II) center
[Cu3]. The coordination environment for Cu1 and Cu2 is satisfied
by a Schiff base ligand in a cis fashion with respect to the phenolate
moieties, while for Cu3 both Schiff base and dca ligands are in-
volved. Thus the terminal Cu1(L) and Cu2(L) sub-units are con-
Observed data [I > 2
Data/restraints/parameters
Final R indices [I > 2
r
(I)]
8389
10794/0/631
R1 = 0.0405
wR2 = 0.0865
R1 = 0.0632
wR2 = 0.0936
1.015
r
(I)]a
R indices (all data)a
Goodness-of-fit on F2
Residuals (e Åꢁ3
)
0.426; ꢁ0.716
nected to each other through Cu3 via double l2-phenoxo bridges.
a
R =
R
(|Fo ꢁ Fc|)/
R
|Fo|, wR = {
R
[w(|Fo ꢁ Fc|)2]/
R .
[w|Fo|2]}1/2
The coordination environment of Cu1 and Cu2 is satisfied by the
phenoxo oxygen and imine nitrogen atoms of the Schiff base li-
gands [Cu1; O101, O123, N110, N114 and Cu2; O201, O223,
corresponds to the mC@N group of the coordinated ligand. The
stretching band of phenolic-OH group of the Schiff base ligand, ob-
served at 3449 cmꢁ1, was absent in the complex, indicating its
deprotonation followed by coordination to the metal ion. Ligand
coordination was also substantiated by the lowering of the pheno-
lic C–O stretching band, which appeared at 1164 cmꢁ1 in 1, other-
wise observed at 1204 cmꢁ1 in the free Schiff base ligand [41,42].
Coordination of the Schiff base ligand was further confirmed by
the appearance of two bands at 420 and 365 cmꢁ1 corresponding
N210, N214], while that for Cu3 is satisfied by four
l2-phenoxo
oxygen atoms [O101, O123, O201, O223] and two dca nitrogen
atoms N1 and N5⁄ (symmetry related counter part of N5). The four
coordinated geometry around Cu1 and Cu2 is considered to be
to mCu–N and mCu–O respectively [43]. Coordination of the dca anion
of NaN(CN)2 was suggested by the appearance of three sharp and
strong characteristic stretching frequencies in the 2300–
2160 cmꢁ1 region. These bands are attributed to mas
+
ms(C–N) com-
bination modes,
mas(C„N) and ms(C„N) vibrations, and are ob-
served at 2292, 2240 and 2180 cmꢁ1 in 1 compared to the bands
at 2286, 2232 and 2179 cmꢁ1 usually observed for free dca. The
shifts toward higher frequencies of these peaks as compared to free
dca are due to the bridging modes of dca in 1. Furthermore, the
absorption bands for
mas(C–N) and ms(C–N) stretching frequencies,
usually observed in the range 1400–1300 and 950–900 cmꢁ1 for
l
1,5-dca bridged copper(II) complexes [44–46], appeared at 1350
and 940 cmꢁ1 respectively in 1. The presence of the perchlorate an-
ion was indicated by a broad trifurcated band appearing in the
range 1140–1100 cmꢁ1 of the spectrum of 1.
3.2. Electronic spectrum
The electronic spectrum of 1 shows two bands in the regions
455–330 and 295–205 nm. The lower energy band corresponds
to the n ? p⁄ transition of the imine group, while the higher en-
ergy band corresponds to the benzene
p ?
p⁄ transition of the
Schiff base ligand (intra-ligand charge transfer) [29]. The electronic
spectrum of 1 also shows an intense band in the high energy region
at 300–372 nm, which can be assigned to charge transfer L ? M
(LMCT) bands. Although the specific nature of this transition
involving the phenolate group is not clearly understood [either
Fig. 1. An ORTEP view of the asymmetric unit of 1 with atom labelling (thermal
ellipsoids are drawn at the 30% probability level, hydrogen atoms and the
perchlorate ion are omitted for clarity).

P. Talukder et al. / Polyhedron 30 (2011) 1767–1773
1771
Fig. 2. Hexanuclear view of the complex ion present in 1.
slightly distorted square-planar, as evident from the relative vari-
ations in bond lengths and angles (Table 2). The Cu–O and Cu–N
bond lengths for Cu1 and Cu2 vary in the range 1.8891(18)–
1.9318(18) and 1.9530(2)–1.9850(2) Å, respectively, are in good
agreement with related reported complexes [29,30]. Cisoid and
transoid angles for both Cu1 and Cu2 also deviate from their ideal
values of 90° and 180° and found to be in the range 78.42(7)–
98.42(9)° and 162.51(8)–168.39(9)°, respectively. The copper(II)
ions are deviated out of the square plane by 0.026 and 0.037 Å,
while the Schiff base donor atoms remain almost coplanar within
0.09 and 0.24 Å with respect to the mean square planes around
Cu1 and Cu2, respectively.
connections are not similar. It is bridged via the axial phenoxo oxy-
gen [O223] and equatorial phenoxo oxygen [O201] to Cu2, but by
two equatorial oxygen atoms [O101 and O123] to Cu1. The mean
equatorial plane of the octahedron is formed by the O101, O123,
O201 and N5⁄ atoms, with N1 and O223 atoms at the two apexes.
The equatorial and axial bond lengths vary in the range
1.9691(17)–2.0473(16) and 2.218(2)–2.5590(19) Å, respectively,
the latter being sensibly longer compared to the former, indicating
the tetragonally elongated octahedral geometry of Cu3, evident of
Jahn–Teller distortion with the Cu(II) d9 ion. The cisoid and transoid
angles in the mean equatorial plane are in the range 74.03(6)–
111.17(8) and 145.60(8)–163.38(9)°, while the trans-axial angle
(N1–Cu3–O223) is 168.66(7)°. The high value of the trans-axial an-
gle compared to equatorial transoid angles may be due to the strain
imposed by the bulky Schiff base ligands. Cu3 is displaced by
0.357 Å towards the apical N1 atom from the mean equatorial
plane. The Cu–O(phenoxo)–Cu angles within the asymmetric unit
vary in the range 89.48(7)–107.54(9)°, where the Cu2–O201(phen-
oxo)–Cu3 angle is sensibly larger (107.54(9)°) compared to the
other Cu–O(phenoxo)–Cu angles in the unit. The metal–metal sep-
arations within the asymmetric unit are found to be 2.957
(Cu1ꢀ ꢀ ꢀCu3) and 3.167 Å (Cu2ꢀ ꢀ ꢀCu3). The Cu1–Cu2–Cu3 angle of
74.58° indicates the non-linear arrangement of copper(II) ions
within the asymmetric unit.
On the other hand, the central copper(II) [Cu3] of the asymmet-
ric unit is six coordinated and displays a distorted octahedral
geometry. Though Cu3 atom is interconnected with the terminal
Cu1 and Cu2 atoms via double
l-phenoxo bridges, the mode of
Table 2
Selected bond lengths (Å) and bond angles (°) for 1.
Bond lengths (Å)
Cu1–O101
Cu1–O123
Cu1–N110
Cu1–N114
Cu2–O201
Cu2–O223
Cu2–N210
Cu2–N214
Cu3–O101
1.9318(18)
1.9116(18)
1.962(2)
Cu3–O123
Cu3–O201
Cu3–O223
Cu3–N1
Cu3–N5a
N1–C2
C2–N3
N3–C4
C4–N5
2.0473(16)
1.9691(17)
2.5590(19)
2.218(2)
1.978(2)
1.154(3)
1.308(4)
As the molecule lies on an inversion centre, the coordination
environment as well as the geometry of the copper(II) centers in
the symmetry related counterpart are exactly the same. Moreover,
these symmetry related units are connected to each other through
1.953(2)
1.9571(18)
1.8891(18)
1.985(2)
1.962(2)
1.9689(18)
1.301(4)
1.154(3)
Cu3 and Cu3⁄ atoms via double
l1,5-bridges, forming a very stable
close and puckered twelve member ring in a chair form incorporat-
ing the Cu3–N1–C2–N3–C4–N5–Cu3⁄–N1⁄–C2⁄–N3⁄–C4⁄–N5⁄
atoms. The bond angles related to the bridging dca ligands are
Bond angles (°)
O101–Cu1–O123
O101–Cu1–N110
O101–Cu1–N114
O123–Cu1–N114
O123–Cu1–N110
N110–Cu1–N114
O201–Cu2–O223
O201–Cu2–N210
O201–Cu2–N214
O223–Cu2–N210
O223–Cu2–N214
N210–Cu2–N214
O101–Cu3–O123
O101–Cu3–O201
O101–Cu3–O223
O101–Cu3–N1
78.42(7)
89.10(8)
168.39(9)
92.13(8)
166.85(8)
100.74(9)
85.09(8)
88.87(9)
167.44(8)
162.51(8)
90.67(9)
98.42(9)
74.42(7)
145.60(8)
78.49(7)
111.17(8)
O101–Cu3–N5a
O123–Cu3–O201
O123–Cu3–O223
O123–Cu3–N1
O123–Cu3–N5a
O201–Cu3–O223
O201–Cu3–N1
O201–Cu3–N5a
O223–Cu3–N1
O223–Cu3–N5a
N1–Cu3–N5a
Cu2–O223–Cu3
Cu2–O201–Cu3
Cu1–O123–Cu3
Cu1–O101–Cu3
C2–N3–C4
91.04(8)
86.60(7)
74.03(6)
102.25(8)
163.38(9)
68.70(7)
100.63(8)
101.82(8)
168.66(7)
95.51(8)
90.37(9)
89.48(7)
107.54(9)
96.57(7)
98.56(8)
121.7(2)
134.0(2) (Cu3–N1–C2) and 158.8(2)° (Cu3–N5⁄–C4⁄) with
a
Cu3ꢀ ꢀ ꢀCu3⁄ separation of 7.043 Å. The Cu–NCNCN–Cu bridging ar-
ray adopts a ‘‘V’’ type conformation because of the sp2 hybridiza-
tion of the middle nitrogen [N3] atom of dca, where the C–N–C
angle (C2–N3–C4) is 121.7(2)° [49]. An approximate C2V symmetry
is observed for the dca ligands, with the N1–C2 and C4–N5 bond
distancesꢁboth being 1.154(3) Å, as commonly observed for the
[N(CN)2]2 anion.
A small degree of p conjugation within
[N(CN)2]ꢁ results in longer C2–N3 (1.309(3) Å) and C4–N3
(1.301(3) Å) bond distances.
The hydrogen bonding parameters for 1 are summarized in
Table 3. The complex possesses a classical hydrogen bond of the
O–Hꢀ ꢀ ꢀO type within the 2-hydroxyacetophenone molecules
a
Symmetry codes = ꢁx, ꢁy, ꢁz.

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P. Talukder et al. / Polyhedron 30 (2011) 1767–1773
Table 3
Hydrogen bonding parameters (Å, °) for 1.
D–Hꢀ ꢀ ꢀA
d(D–H) (Å) d(Hꢀ ꢀ ꢀA) (Å) d(Dꢀ ꢀ ꢀA) (Å) \(D–Hꢀ ꢀ ꢀA) (°)
O301–H1ꢀ ꢀ ꢀO310
0.85(4)
0.98
1.79(4)
2.62
2.56
2.46
2.53
2.49
2.58
2.48
2.46
2.574(3)
3.505(4)
3.503(3)
3.431(3)
3.299(4)
3.293(3)
3.375(3)
3.285(4)
3.397(4)
153(4)
150
161
171
135
139
137
138
169
C109–H10Aꢀ ꢀ ꢀN3i
C109–H10Bꢀ ꢀ ꢀO310ii 0.98
C109–H10Cꢀ ꢀ ꢀO2iii
C116–H11Iꢀ ꢀ ꢀO2
C209–H20Cꢀ ꢀ ꢀO3iv
C211–H21Bꢀ ꢀ ꢀO3v
C213–H21Fꢀ ꢀ ꢀO3
C120–H120ꢀ ꢀ ꢀO1
0.98
0.98
0.98
0.99
0.99
0.95
Fig. 4. Schematic diagram of the trinuclear asymmetric unit with donor atoms,
showing Cuꢀ ꢀ ꢀCu separations and designated exchange couplings (2J).
Symmetry transformations used to generate equivalent atoms: (i) ꢁ1 + x, y, z (ii) ꢁx,
1 ꢁ y, 1 ꢁ z; (iii) ꢁx, ꢁy, 1 ꢁ z (i ) 1 ꢁ x, ꢁy, ꢁz ( ) 1 + x, y, z.
v
v
coupling is occurring, such that
v
T (per Cu3) decreases from
present in the lattice. However, several non-classical C–Hꢀ ꢀ ꢀO
interactions are also present. The closest Cꢀ ꢀ ꢀO distance is 2.46 Å.
The presence of two 2-hydroxyacetophenone molecules in the lat-
tice may be inferred from the hydrolysis of the Schiff base ligand
(H2L) under the reaction conditions.
0.683 cm3 molꢁ1
K
at 300 K to reach
a
plateau value of
0.465 cm3 molꢁ1 K between ca. 100 and 4.2 K. This plateau is indic-
ative of an unpaired spin (per trinuclear moiety) in the ground
state. Because there is a centre of inversion in the Cu6 cluster, a
‘‘pair of independent triangles’’ model was employed to fit the data
[35] and the best fit is shown as a solid line in Fig. 3. Such a model
3.4. Electrochemical study
is reasonable in view of the very weak l1,5-dca coupling expected
[53] between each Cu3 half of the hexanuclear cluster, separated by
l-Phenoxo bridging is known to yield strong
antiferromagnetic coupling, particularly when copper coplanar
Cu3ꢀ ꢀ ꢀCu3⁄ = 7.043 Å.
The cyclic voltammogram of 1 is recorded in the potential range
ꢁ1.2 to +1.2 V. On cathodic scanning, it shows two peaks at ꢁ0.70
and ꢁ1.02 V (Epc) (versus SCE). These reduction potentials corre-
spond to the CuII/CuI process of each of the two different types of
copper(II) centers and the observed peak-to-peak separation of
320 mV indicates the irreversible nature of the reductions, which
is believed to be due to the combined effect of the difference in
coordination environment around the two types of copper(II) cen-
ters (square planar for Cu1 and Cu2; octahedral for Cu3) and the
equatorial [CuN2O2] planes are bridged by two
l-PhO groups
[54]. This coplanarity does not exist in each half of the present
cluster, the central copper(II) [Cu3] being bridged via an axial
(2.5590(17) Å) and an equatorial (1.9691(17) Å) phenoxo oxygen
to one of the terminal copper(II) atom [Cu2] (Cu2ꢀ ꢀ ꢀCu3 = 3.167 Å)
but by two equatorial oxygens (Cu–O = 1.9689(16) and
2.0473(17) Å)
to
the
other
copper(II)
atom
[Cu1]
effective communication between them through the double l2
-
(Cu1ꢀ ꢀ ꢀCu3 = 2.957 Å). The terminal copper(II) atoms are not
bridged directly, but are connected via the central Cu3, and are
separated by 3.714 Å.
The ‘‘pair of independent triangles’’ model yielded a good fit for
the parameter values g = 2.21, temperature independent paramag-
netic susceptibility (TIP) = 200 ꢂ 10ꢁ6 cm3 molꢁ1 for Cu3,
2JAB=AC = ꢁ407 cmꢁ1 and 2JBC = 0 (Fig. 4). A difference in 2J for the
Cu1–Cu3 versus Cu2–Cu3 pathways could not be obtained with
certainty. 2J values of this size and magnitude are quite common
phenoxo bridges. These observations are consistent with the
behavior found in some related macrocyclic doubly phenoxo-
bridged dicopper complexes, which show two well-separated one
electron reductions [50–52]. On anodic scanning, only one oxida-
tive response is observed at +0.81 V (versus SCE). The process is
tentatively assigned to the oxidation of the coordinated ligand
[L]2ꢁ, as the cyclic voltammogram of the free ligand [30] shows
only one oxidative response at +0.76 V. The results suggest that
the CuI complex is unstable and undergoes rapid decomposition
upon reduction.
for Cu(l-OPh)2Cu pathways [54].
4. Conclusion
3.5. Magnetic properties
In the present paper we have reported the synthesis and phys-
icochemical characterization of a novel hexanuclear copper(II)
Schiff base complex, derived from a symmetrical N2O2 donor Schiff
The plot of
vT versus temperature for 1, obtained in a field of
1 T, is shown in Fig. 3. It indicates that strong antiferromagnetic
base and dicyanamide ion.
l1,5-Bridging mode of dicyanamide is
0.9
0.8
0.7
0.6
0.5
0.4
0.3
explored to form a complex of six copper(II) centers through dou-
bly bridging between two trimetallic asymmetric units. Structural
characterization also reveals both distorted square planar and dis-
torted octahedral copper(II) centers within the asymmetric unit
where each terminal copper(II) is connected to the middle cop-
per(II) by double l2-phenoxo bridges. Cryomagnetic investigation
reveals strong antiferromagnetic behavior well fitted with the
‘‘pair of independent triangles’’ model [35] as the long chain dca
generally exhibits no or a very weak antiferromagnetic interaction.
Acknowledgement
50
100
150
200
250
300
T / K
P.T. and S.M. gratefully acknowledge the financial support from
the Council of Scientific and Industrial Research (CSIR), New Delhi,
Government of India.
Fig. 3. Thermal variation of
best fit to the model.
vT (per trinuclear unit) in 1. Solid lines represent the

P. Talukder et al. / Polyhedron 30 (2011) 1767–1773
1773
[24] J. He, J.-X. Zhang, C.-K. Tsang, Z. Xu, Y.-G. Yin, D. Li, S.-W. Ng, Inorg. Chem. 47
(2008) 7948.
Appendix A. Supplementary data
[25] T. Shiga, K. Maruyama, L. Han, H. Oshio, Chem. Lett. 34 (2005) 1648.
[26] M. Murugesu, R. Clerac, C.E. Anson, A.K. Powell, Chem. Commun. (2004) 1598.
[27] M. Murugesu, R. Clerac, C.E. Anson, A.K. Powell, Inorg. Chem. 43 (2004) 7269.
[28] A. Datta, C.R. Choudhury, P. Talukder, S. Mitra, L. Dahlenburg, T. Matsushita, J.
Chem. Res. (S) (2003) 642.
[29] S. Thakurta, J. Chakraborty, G. Rosair, J. Tercero, M.S. El Fallah, E. Garribba, S.
Mitra, Inorg. Chem. 47 (2008) 6227.
[30] A. Ray, S. Mitra, A.D. Khalaji, C. Atmani, N. Cosquer, S. Triki, J.M. Clemente-Juan,
S. Cardona-Serra, C.J. Gomez-Garcia, R.J. Butcher, E. Garribba, D. Xu, Inorg.
Chim. Acta 363 (2010) 3580.
CCDC 799223 contains the supplementary crystallographic data
for 1. These data can be obtained free of charge via http://
www.ccdc.cam.ac.uk/conts/retrieving.html, or from the Cambridge
Crystallographic Data Centre, 12 Union Road, Cambridge CB2 1EZ,
UK; fax: (+44) 1223-336-033; or e-mail: deposit@ccdc.cam.ac.uk.
References
[31] T.K. Karmakar, S.K. Chandra, J. Ribas, G. Mostafa, T.-H. Lu, B.K. Ghosh, Chem.
Commun. (2002) 2364.
[32] U. Ray, S.K. Jasimuddin, B.K. Ghosh, M. Monfort, J. Ribas, G. Mostafa, T.H. Lu, C.
Sinha, Eur. J. Inorg. Chem. (2004) 250.
[33] T.K. Karmakar, B.K. Ghosh, A. Usman, H.-K. Fun, E. Riviere, T. Mallah, G. Aromí,
S.K. Chandra, Inorg. Chem. 44 (2005) 2391.
[34] T.K. Karmakar, G. Aromí, B.K. Ghosh, A. Usman, H.-K. Fun, T. Mallah, U. Behrens,
X. Solans, S.K. Chandra, J. Mater. Chem. 16 (2006) 278.
[1] O. Kahn, Molecular Magnetism, VCH, New York, 1993.
[2] D. Gatteschi, O. Kahn, J.S. Miller, F. Palacio, Molecular Magnetic Materials,
NATO ASI Series, Kluwer, Dordrecht, The Netherlands, 1991.
[3] E. Coronado, P. Delhaès, D. Gatteschi, J.S. Miller, Molecular Magnetism: From
Molecular Assemblies to the Devices, Kluwer, Dordrecht, The Netherlands,
1996.
[4] D. Gatteschi, A. Caneschi, R. Sessoli, A. Cornia, Chem. Soc. Rev. 25 (1996) 101.
[5] B. Le Guennic, S. Petit, G. Chastanet, G. Pilet, D. Luneau, N. Ben Amor, V. Robert,
Inorg. Chem. 47 (2008) 572.
[35] L. Spiccia, B. Graham, M.T.W. Hearn, G. Lazarev, B. Moubaraki, K.S. Murray,
E.R.T. Tiekink, J. Chem. Soc., Dalton Trans. (1997) 4089.
[6] T. Mallah, M.L. Boillot, O. Kahn, J. Gouteron, S. Jeannin, Y. Jeannin, Inorg. Chem.
25 (1986) 3058.
[36] XSCANS, Data Collection and Reduction Program, Version 2.2, Siemens Analytical
X-ray Instruments Inc., Madison, WI, 1994.
[7] T. Mallah, O. Kahn, J. Gouteron, S. Jeannin, Y. Jeannin, C.J. O’Connor, Inorg.
Chem. 26 (1987) 1375.
[8] O. Kahn, T. Mallah, J. Gouteron, S. Jeannin, Y. Jeannin, J. Chem. Soc., Dalton
Trans. (1989) 1117.
[9] A. Benzekri, P. Dubourdeaux, J.M. Latour, J. Laugier, P. Rey, Inorg. Chem. 27
(1988) 3710.
[37] G.M. Sheldrick, SADABS, Program for Empirical Absorption Correction of Area
Detector Data, University of Göttingen, Göttingen (Germany), 1996.
[38] G.M. Sheldrick, SHELXTL version 5.1: Program for the Solution and Refinement of
Crystal Structures, Bruker AXS P4 Inc., Madison, WI, USA, 1999.
[39] G.M. Sheldrick, SHELXS/L-97, Programs for Crystal Structure Determination,
University of Göttingen, Göttingen (Germany), 1997.
[10] T. Chattopadhyay, K.S. Banu, A. Banerjee, J. Ribas, A. Majee, M. Nethaji, D. Das, J.
Mol. Struct. 833 (2007) 13.
[40] L.J. Farrugia, J. Appl. Cryst. 30 (1997) 565.
[41] K. Nakamoto, Infrared and Raman Spectra of Inorganic and Coordination
Compounds, Theory and Applications in Inorganic Chemistry, fifth ed., John
Wiley, 1997.
[42] Z.-L. You, H.-L. Zhu, Z. Anorg. Allg. Chem. 630 (2004) 2754.
[43] S. Thakurta, P. Roy, G. Rosair, C.J. Gómez-García, E. Garribba, S. Mitra,
Polyhedron 28 (2009) 695.
[11] P. Chaudhuri, R. Wagner, T. Weyhermuller, Inorg. Chem. 46 (2007) 5134.
[12] J. Yoon, E.I. Solomon, Coord. Chem. Rev. 251 (2007) 379.
[13] T. Glaser, M. Heidemeier, J.B.H. Strautmann, H. Bogge, A. Stammler, E.
Krickemeyer, R. Huenerbein, S. Grimme, E. Bothe, E. Bill, Chem. Eur. J. 13
(2007) 9191.
[14] B. Sarkar, M.S. Ray, Y.-Z. Li, Y. Song, A. Figuerola, E. Ruiz, J. Cirera, J. Cano, A.
Ghosh, Chem. Eur. J. 13 (2007) 9297.
[44] J. Carranza, C. Brennan, J. Sletten, F. Lloret, M. Julve, J. Chem. Soc., Dalton Trans.
(2002) 3164.
[15] M. Casarin, C. Corvaja, C. Di Nicola, D. Falcomer, L. Franco, M. Monari, L.
Pandolfo, C. Pettinari, F. Piccinelli, Inorg. Chem. 44 (2005) 6265.
[16] Y.-Z. Zhang, H.-Y. Wei, F. Pan, Z.-M. Wang, Z.-D. Chen, S. Gao, Angew. Chem.,
Int. Ed. 44 (2005) 5841.
[17] W. Ouellette, A.V. Prosvirin, V. Chieffo, K.R. Dunbar, B. Hudson, J. Zubieta,
Inorg. Chem. 45 (2006) 9346.
[18] C. Yuste, A. Bentama, S.-E. Stiriba, D. Armentano, G. De Munno, F. Lloret, M.
Julve, J. Chem. Soc., Dalton Trans. (2007) 5190.
[19] K.C. Mondal, P.S. Mukherjee, Inorg. Chem. 47 (2008) 4215.
[20] C.-D. Wu, L. Ma, W. Lin, Inorg. Chem. 47 (2008) 11446.
[21] S.-B. Ren, X.-L. Yang, J. Zhang, Y.-Z. Li, Y.-X. Zheng, H.-B. Du, X.-Z. You, Cryst.
Eng. Commun. 11 (2009) 246.
[45] G.A. van Albada, I. Mutikainen, U. Turpeinen, J. Reedijk, Acta Cryst. E57 (2001)
m421.
[46] M. Du, X.-J. Zhao, S.R. Batten, J. Ribas, Cryst. Growth Des. 5 (2005) 901.
[47] A. Golcu, M. Tümer, H. Demirelli, R.A. Wheatley, Inorg. Chim. Acta 358 (2005)
1785.
[48] A.B.P. Lever, Inorganic Electronic Spectroscopy, Elsevier, New York, 1984.
[49] L.-Y. Zhang, L.-X. Shi, Z.-N. Chen, Inorg. Chem. 42 (2003) 633.
[50] R.R. Gagne, L.M. Henling, T.J. Kistenmacher, Inorg. Chem. 19 (1980) 1226.
[51] C. Fraser, B. Bosnich, Inorg. Chem. 41 (2002) 1788.
[52] K.R. Krishnapriya, M. Kandaswamy, Polyhedron 24 (2005) 113.
[53] S.R. Batten, K.S. Murray, Coord. Chem. Rev. 246 (2003) 103.
[54] W. Mazurek, A.M. Bond, K.S. Murray, M.J. O’Connor, A.G. Wedd, Inorg. Chem.
24 (1985) 2484.
[22] P. Klufers, J. Schuhmacher, Angew. Chem., Int. Ed. 34 (1995) 2119.
[23] L.N. Dawe, L.K. Thompson, Angew. Chem., Int. Ed. 46 (2007) 7440.